This invention relates to a machine for forming on a workpiece a curved surface. The invention has been developed primarily for use in the manufacture of optical lenses, but may also be applied to machines for use in other cutting operations, for example in the formation of a surfacing tool for an optical lens.
The curved surfaces of optical lenses are at present generally formed by cylindrical cutting tools. The cylindrical tool is supported with its axis inclined at an acute angle to the axis of the lens blank and the tool is rotated about its own axis to perform the cutting action and is traversed across the surface of the blank. The curvature of the surface formed on the lens blank by the tool is dependent upon the diameter of the cylindrical tool and also upon the angle at which the axis of the tool is inclined to the axis of the blank. By suitable adjustment of the angular relation between the tool and blank, one tool can be used to cut surfaces having different curvatures. However, the range of curvatures which can be cut with a single tool is severely limited. This limits the usefulness of an individual machine by means of which the cutting operation is carried out. In order to cut surfaces having curvatures outside the range which can be achieved by one cutting tool, that tool can be replaced by another tool of different diameter, but such change of cutting tools involves re-setting the machine which occupies several hours of highly skilled labour. A further disadvantage of the procedure hereinbefore mentioned which is generally used for cutting the curved surfaces on optical lenses is that the accuracy with which the shape of the surface produced conforms to the required shape is not high and varies from one part of the surface to another, the accuracy being greatest in a central region around the optical axis of the lens produced and decreasing with distance away from the axis. Also, the accuracy is greater for surfaces of small curvature and is even less satisfactory for surfaces of severe curvature. These inaccuracies arise partly from the technique of using a cylindrical cutting tool with its axis inclined to the optical axis of the lens and partly from wear of the cutting tool.